FIG. 1 shows a system 100 comprising a subsystem 10 further comprising a radio module 12 and a GPS receiver 13, and a microcontroller 15 for controlling the devices of the subsystem 10. The radio module 12 and the GPS receiver 13 both comprise an interface 121, 131 for command input and data output. Similarly, the microcontroller 15 comprises an interface 151A, 151B for each device of the subsystem 10 for command output and data input.
The microcontroller 15 can be embedded or integrated in or connected to a terminal, for example. If the terminal is to use the radio module 12, it can do so via the microcontroller 15 wherein applications for interfacing the radio module 12 are executed. The radio module 12 comprises a base band unit 123 that is adapted to execute applications for using the radio frequency part 125. The base band unit 123 comprises an internal real time clock 256. Then the necessary communication between the microcontroller 15 and the radio module 12, in order to create a speech or data communication, for example, is carried out between the serial interface 151A in the microcontroller 15 and the serial interface 121 in the radio module 12.
For a satellite positioning functionality, a GPS receiver 13 is connected in a similar manner like the radio module 12, via its serial interface 131 to the other serial interface 151B of the microcontroller 15. The GPS receiver 13 further comprises a radio frequency part 135 and a GPS base band unit 133.
Traditionally, the local data interface of a GPS receiver complies with one of the NMEA standards. NMEA standards NMEA-0180, 0182 and 0183 define electrical interfaces and data protocols for communications between marine instrumentation, for example. These NMEA standards recommend that for a GPS receiver, the interface should comply with RS-232 or EIA-422. In practice, as a consequence, the data connection between a GPS receiver and a device communicating with the GPS receiver requires one data line only and can be carried out using one connector.
The interfaces through which the radio module 12 and of the GPS receiver 13 are controllable by the microcontroller 15 are serial interfaces. For communication, usually the standardized AT command language is used between the microcontroller 15 and the radio module 12. The command language of the GPS receiver 13 is usually the NMEA protocol.
If the device using the system 100 comprising the subsystem 10 and microcontroller 15 needs to use both the radio module 12 and the GPS receiver 13, it would, on its application level, need to process and synchronize data from and to both of these devices. Such a case is encountered, for example, when Assisted GPS is used where a cellular network sends synchronization information, using which the necessary synchronization of the GPS receiver 13 can be performed essentially faster.
The synchronization information is first received from the cellular network by the radio module 12 which then passes it either through the microcontroller 15 to an application running in the terminal or to an application running in the microcontroller 15. The application then in turn passes this information to the GPS receiver 13 through the microcontroller 15.
The synchronization information for a GPS receiver 13 comprises many different kinds of data to enable the GPS receiver 13 to start with the positioning. This information includes in addition to current visibility of satellites also the current date and time. For this reason a real time clock is necessary. A simple GPS receiver 13 nevertheless does not contain an internal real time clock, so the time information has to be available from the microcontroller 15, e.g. from its internal real-time clock 156.